BEIJING, April 2 (Reuters) - Authorities in a southern Chinese city grappling with protests against plans to build a petrochemical plant have detained eight protesters, two of the demonstrators told Reuters on Wednesday.

Since Sunday, hundreds of residents of the town of Maoming have held protests against the plant, for fear it will contribute to pollution. The protesters have complained that violence by law enforcement officers led to deaths and injuries.

Images of protest violence that surfaced on Chinese social media were later deleted by censors, triggering an outcry. The government has said no one was killed in the demonstrations, but it has not said if anyone was hurt.

On Wednesday, a demonstrator who gave his surname as Xi, said he saw eight people being detained in Guangzhou, the provincial capital.

"The police took eight of them away," said Xi. "I think it's a bit excessive. They were simply showing their opinions peacefully."

The plant, to be owned by the local government and China's biggest refiner, state-controlled Sinopec Corp, will churn out paraxylene, a petrochemical used to make fabrics and plastic bottles.

Choking smog blankets many Chinese cities, and environmental degradation, the cost of the country's breakneck economic growth, has earned the ire of an increasingly educated and affluent urban class.

A video of Sunday's protest, which turned violent, showed police in riot gear, wielding batons and chasing dozens of demonstrators through a street, before firing tear gas.

Many protesters travelled to Guangzhou on Tuesday, witnesses told Reuters.

Unverifiable mobile phone videos, obtained by Reuters, showed demonstrators arguing with police, as they held colourful umbrellas and signs demanding that the plant "roll out of Maoming". In one, several police officials pulled away a woman protester.

On Wednesday, the city government said officials had held peaceful talks with protesters the previous day, but it did not acknowledge the complaints about the violence..

Maoming's deputy mayor told protesters the project was still in the planning stages and far from being approved, the city government said in its statement.

However, instead of making concessions on the plans, officials simply tried to convince the protesters that paraxylene was harmless, Xi said.

U.S.-based Human Rights Watch has urged China to investigate the apparent excessive use of force by police.

"Premier Li Keqiang has pledged to launch a 'war on pollution'," China director Sophie Richardson said in a statement on Tuesday. "Yet when citizens demonstrate their concerns for the environment they appear to be in harm's way."

On Wednesday, Hong Lei, a spokesman of the Ministry of Foreign Affairs, said he was not aware of the statement by the rights body, but added, "I know this organisation is usually biased against China." (Editing by Clarence Fernandez)